Many techniques are available for the deposition of thin and thick films of a large variety of materials on a large variety of substrates. For the successful transmission of the required coating material to the substrate, especially under "clean" conditions required for good bonding for many combinations, techniques have been developed for deposition under vacuum conditions. Among these techniques are evaporation, cathode sputtering, rf sputtering, ion plating, vapor decomposition and ion beam sputtering (see e.g. U.S. Pat. No. 3,472,751). In general these are relatively "thin" film techniques as opposed to "thick" film techniques such as painting, cladding, plating, etc.
In spite of this multiplicity of methods, there still exist many combinations of substrates and coating materials which cannot be successfully bonded to produce the required characteristics. Four of the primary reasons for such failures are:
1. Low melting or deformation point of the substrate material, restricting deposition conditions. PA1 2. Softness (i.e., low binding energy) of the substrate material, reducing bonding strength. PA1 3. Large differences in thermal expansion coefficients of the substrate and coating materials, restricting the useful temperature range of the combination to a narrow range around the deposition temperature unless the bond strength is very high. PA1 4. Substrate is hard to clean, resulting in poor bond strength.
An excellent example of all four of these difficulties is given by the problem of applying a hard coating to plastics in order to provide scratch resistance. This is particularly desirable in the case of plastic eyeglass lenses, which are inherently safe to wear since they do not shatter but which easily become scratched and quickly reach a hazy condition requiring their replacement. The safety feature of such plastic lenses could be retained and the useful lifetime extended if it were possible to deposit a "hard" coating on the lens surface to protect it from scratching. In this context "hard" is taken to mean a coating of a material at least as hard as the normal optical glass used for eyeglasses. The plastics, however, used for eyeglass lenses (such as PC39, or CR39), although hard by plastics standards, are inherently difficult to clean and are soft by comparison with many other materials such as metals, glass or semiconductors to which coatings are conventionally applied, thereby making good bonds difficult to achieve. Also they may be worked only at low temperatures (maximum temperature approximately 70.degree. C or less) and they have very high coefficients of expansion (approximately 8-9 .times. 10.sup.-5 /.degree. C) making them incompatible with most "hard" materials which must be worked at high temperatures and which have relatively small coefficients of expansion (less than 1.times.10.sup.-5 /.degree. C). In fact, fused quartz (SiO.sub.2) which has a Mohs hardness of 7 and which is transparent in the visible spectrum would make an excellent protective coating for eyeglass lenses except that it has a coefficient of linear expansion of only 0.54.times.10.sup.-6 /.degree. C.
Basically it is desirable to use plastic to make eyeglass lenses because of the ease with which the basic blanks can be molded and because of their safety features, but it is desirable to apply a scratch resistant coating to extend the life. Many other similar applications are immediately obvious such as hard coatings for plastic safety windows, plastic instrument lenses (cameras, binoculars, etc.), plastic auto and airplane windows, etc. In addition, it is desirable to apply colored or reflecting layers to plastics for light control (sunglasses), for heat control (windows), for aesthetic reasons (decorative panels) etc., and also to apply hard protective coatings over such optical layers. Similar applications exist with other materials (e.g. applying solderable layers to boron nitride crystals, etc.). For purposes of the present invention, the eyeglass lens problem is intended to be illustrative only and is not intended to be limiting.
Coating materials such as organic compounds, which can be applied under low temperature non-vacuum conditions, at best provide marginal improvement over the basic plastic used in the lenses and do not compete in hardness with materials such as fused quartz or even common glass. In order to put down the latter at low temperatures, vacuum techniques must be used and even here most of the conventional processes do not work at low enough substrate temperatures to be compatible with plastics. The process of the present invention does, however, operate at the necessary low temperature and can be extended to large volume commercial production. It is a modified form of ion beam sputtering (for a description of the basic process, see U.S. Pat. No. 3,472,751).